The invention relates to stimulation of nerves by pulsed magnetic fields. Such fields induce in the body of an exposed subject eddy currents that are proportional to their rate of change. The currents may cause classical nerve stimulation wherein the nerve membrane is depolarized enough for the nerve to fire. At low frequencies, such a mechanism requires rather large magnetic fields. Fortunately, low-frequency magnetic manipulation of the nervous system is possible by another mechanism which allows the use of very much weaker fields. Instead of relying on causing the firing of normally quiescent nerves, the method uses modulation of the spiking patterns of spontaneously firing nerves. That this can be done with very small tissue electic fields was discussed more than four decades ago by C. A. Terzuolo and T. H. Bullock in "Measurement of Imposed Voltage Gradient Adequate to Modulate Neuronal Firing", Proceedings of the National Academy of Sciences U.S.A., Physiology, 42, 687 (1956). The effect can be exploited in magnetic as well as in electric stimulation, because the physiological effects of the former are solely due to the electric field that is induced by the rate of change of the magnetic field, and by the electric polarization that occurs as the consequence of the induced eddy currents.
The human nervous system exhibits a sensitivity to certain low-frequency stimuli, as is evident from rocking a baby or relaxing in a rocking chair. In both cases, the maximum soothing effect is obtained for a periodic motion with a frequency near 1/2 Hz. The effect is here called "the 1/2 Hz sensory resonance". In the rocking response, the sensory resonance is excited principally by frequency-coded signals from the vestibular end organ. However, the rocking motion also induces body strains, and these are detected by stretch receptors residing in the skin and elsewhere in the body. In addition, relevant signals may originate from thermal receptors which report skin temperature fluctuations caused by air currents that are induced by the rocking motion. All these receptors employ frequency coding in their sensory function, and it must be that their signals are combined and compared in the brain with the vestibular nerve signals in an assessment of the somatic state. One may thus expect that the sensory resonance can be excited not only through the vestibular nerve, but also separately through the other sensory modalities mentioned. This notion is supported by the observation that gently stroking of a child with a frequency near 1/2 Hz has a soothing effect. Further support derives from the successful excitation of the 1/2 Hz sensory resonance by weak external electric fields, as discussed in "Method and Apparatus for Manipulating Nervous Systems", U.S. Pat. No. 5,782,874. The 1/2 Hz sensory resonance involves the autonomic nervous system, and it can be used to induce relaxation, sleepiness, or sexual excitement, depending on the precise stimulation frequency and the affected afferent nerves. Another sensory resonance has been found at about 2.4 Hz; it involves the cortex since it can slow the speed of silently counting from 100 to 60, with the eyes closed, as discussed in the '874 patent and in U.S. Pat. No. 5,800,481. For both electric field and thermal stimulation, prolonged exposure to fluctuating electric fields near 2.4 Hz has been found to have a sleep-inducing and dizzying effect. The same physiological effect is expected for pulsative magnetic stimulation, since electric fields are induced in the tissue by the changing magnetic field. When using the nerve modulation method, reliance on resonance mechanisms further reduces the stimulation strength required for manipulating the nervous system.